This invention relates to screen printing presses in general and, in particular, to means for driving a squeegee and flood bar across a printing screen and reversing their position during a press cycle.
In a conventional screen printing operation, during a printing cycle, it is normally necessary to push a pool of ink across the screen toward one end of it, in a "flood stroke," then to place the screen immediately on top of the work, lift the flood bar and lower a squeegee bar extending across the width of the screen into contact with the screen. The rubber squeegee is then drawn in the opposite direction across the screen to force the ink through the screen in the areas desired and thereby print the work.
Various means have been devised to drive a squeegee and a flood bar, mounted on what is often called the "carriage," across the screen and to reverse their positions during the press cycle.
A few of these methods have dealt with the problem of timing the movement of the squeegee and flood bar relative to the screen and to one another during an automated cycle in which the printing head, including the carriage assembly and the chase supporting the screen, are pivoted relative to the printing bed to allow insertion and removal of stock for printing. Gorner (U.S. Pat. No. 1,821,302), Landesman (U.S. Pat. No. 2,894,451) and Walsh, Jr. (U.S. Pat. No. 2,704,510) are disclosed various screen printing machines having apparatus for moving a squeegee and/or flood bar relative to a printing screen and during a printing cycle. U.S. Pat. No. 3,731,623, issued May 8, 1973, to Henry J. Bubley and Claude H. Oltra, and titled "Glider Press" and U.S. Pat. No. 3,859,917 issued Jan. 14, 1975 to American Screen Printing Equipment Company assignee of, Bubley and Oltra and titled "Improvement in a Screen Printing Press," also teach screen printing pressed having apparatus for timed actuation of the squeegee and flood bar. U.S. Pat. No. 3,731,623 and U.S. Pat. No. 3,859,917, which were developed by the same inventors and are assigned to the same assignee as the present invention, are hereby specifically incorporated in the present application in their entireties by reference thereto.
Through application U.S. Pat. No. 3,859,917 contains many unique and desirable features which make it a distinct improvement over the presses of the prior art, including a second frame which pivots, in its entirety, relative to the support frame about pivot points mounted in the plane of and at the rear of the screen, and which also provides for automatic, timed movement of the carriage assembly relative to the angular movement of the printing head, it still, particularly in smaller size presses, has some disadvantages.
The press shown in application U.S. Pat. No. 3,859,917 has a pair of extremely large generally L-shaped drive arms mounted on the support frame, which are connected through connecting rods to opposite ends of the carriage assembly. The carriage assembly is actuated by a shuttle which is allowed to move freely relative to a carriage housing and which operates the squeegee and flood bar mounted on the carriage housing by means of a mounting pin riding in a track formed in the shuttle. When the shuttle contacts bumpers at opposite ends of the support arms, the position of the squeegee and flood bar will be reversed as the mounting pin moves in the shuttle track.
The use of two large, heavy drive arms presents alignment and weight problems and adds to the cost of the presses. The position of each arm relative to their common shaft must be exactly aligned so that the carriage assembly is always perpendicular to its support arms, to assure proper tracking of the carriage relative to the screen. In addition, when the length of the stroke of the carriage assembly is adjusted to correspond to the various sizes of stock to be printed, the bumpers which actuate the shuttle must be removed. If they are not, the carriage may smash into the bumpers and severely damage the head. When these bumpers are moved, they must be carefully aligned in corresponding positions on each arm so that the opposite ends of the squeegee or the flood bar are moved simultaneously. The use of a shuttle or sliding actuator has also resulted in sticking, noise, and high cost of manufacture because of the number of parts. Previous squeegee actuators have also not provided a means of positively locking the squeegee or flood bar in position relative to the fixed frame during the printing stroke.